By suggesting the OP load 20 thou off the lands you are taking him back to where he started, factory loads shoot ok in the rifle its his reloads where the problem lies, possibly originating in this "load as close to the lands as possible" mantra that seems to have taken hold .
The OP has got it into his head that starting at the lands and then backing away untill he finds a sweet spot is the way to go. Hes had no progress with backing of thats why I said go back close to the lands.
Hes been cleaning the barrel before each test string effectivelly turning a test group into fouling shots which will have poor to zero accuracy.

I am guessing/hoping that you worked up a powder load at book COAL then picked the tightest group that didn't show any pressure signs and started to change the length to fine tune
Im with pothunter here.
Reload development starts with powder load testing and then COL for fine tuning